A slag composition comprised of from about 85 to about 99 weight percent of steelmaking slag and from about 1 to about 15 weight percent of a foaming additive. The foaming additive is comprised of from about 20 to about 80 weight percent of a source of elemental carbon, from about 5 to about 80 weight percent of calcium carbide, and from about 25 to about 75 weight percent of an oxide compound. The weight/weight ratio of the source of elemental carbon/calcium carbide is at least 2.
The present invention is directed to providing an optimal composition for achieving desired properties in a steelmaking slag within an electric arc furnace, including the creation of appropriate basicity and chemistry for optimal foaming at each stage of the process, as well as a method and system for accomplishing the optimal composition.
In electric arc melting of steel the energy efficiency and overall cost is greatly improved by the creation of a basic foamy slag within the furnace. Foaming of the slag in order to engulf the electric arc is well known in the art, and is described, for example, in U.S. Pat. No. 4,447,265 and in U.S. Pat. No. 4,528,035. The procedures described involve the injection of carbonaceous material into the furnace slag to cause carbon monoxide gas evolution, and the escaping gas causes the slag to effervesce or foam. As a result of the effervescence or foaming, the slag is increased in its volume, engulfing the electric arcs and improving electrical and heat transfer efficiency.
In U.S. Pat. No. 4,473,397, the inventors discuss the injection of calcium carbide into a steel making convertor to decrease the amount of slag foaming. Contrary to that finding, U.S. Pat. No. 5,279,639 teaches that the introduction of calcium carbide into a slag with high oxygen potential causes slag foaming. Neither patent teaches the use of calcium carbide to create a foamy slag in an electric arc steel melting furnace.
In U.S. Pat. No. 5,279,639 the use of calcium carbide as the reductant in ladle slag treatment is discussed. The invention discloses that calcium carbide is also an effective slag foaming agent due to the release of carbon monoxide during reaction in the slag. This reaction product provides stirring and foaming of the slag with the benefits of mixing, enhanced reaction rates and arc stability, as described in the patent. It was noted that the release of gas was sustained over a period of time, and that foaming was sometimes considered excessive. The furnace slag could benefit from the sustained foaming observed during the use of calcium carbide. The present invention teaches a system and method to use calcium carbide and other materials to advantage in the creation of optimum foamy slag conditions in the electric arc melting furnace.
As described in the prior art, the use of injected oxygen aids in the production of carbon monoxide gas. Also described in the prior art, calcium oxide is sometimes injected along with the carbon and the oxygen in order to raise the slag basicity and stabilize the foam that is created. In addition, other cost benefits are derived such as reduced refractory lining wear, higher productivity, reduced graphite electrode consumption, and increased yield of iron units from the melted scrap. Foamy slag practice is arguably one of the greatest improvements to electric arc steel melting technology of the 20th century.
In the steel melting process, the chemical and physical properties of the slag can influence the economics of production and the quality of the final steel product. The slag is formed from several sources, possibly including added calcium and magnesium oxides, eroded refractory, residual slag from a previous steel batch, alloying elements that are present in the scrap charge that are oxidized during the steelmaking process, gangue materials that are present in the charge carbon or injected carbon materials, and entrained oxides that coat the scrap charge or are otherwise introduced into the process. Since the amount of these contributors to the slag may vary considerably from batch to batch, the chemistry of the slag on top of the steel may be quite variable, and hence the ability to cause and maintain a stable foamy slag may also be quite variable. As a result, the current art of slag foaming in the electric arc steel melting furnace is highly variable in its effectiveness.
The ideal chemistry for optimum slag foaming is different at different times in the process. Initially, while the iron oxide content of the slag is low, a lower basicity slag is required. Conversely, as the process progresses and iron oxide content of the slag increases, a higher basicity slag is better for a sustained foamy slag. Current technology, however, does not recognize or provide for this variance. Several characteristics are required in the furnace slag to optimize slag foaming at all stages of the process:
1. Slag with both fluid and solid components in appropriate ratio
2. Adequate viscosity to retain sufficient fraction of gas for a stable foam
3. Adequate sources of carbon and oxygen to sustain the foam
4. Control of FeO content of the slag by independent control of oxygen and carbon injection rates
5. Gas generation rate sufficient for foaming but not enough to cause foam collapse
6. Non-corrosive to refractory lining
7. Minimum slag weight that can accomplish required slag volume and chemistry
Again, the current state of the art does not consistently and reproducibly provide a foamy slag on each steel batch of steel that optimally meets the above criteria. The amount and composition of indigenous slag components are not accurately known, and the intentionally added components are typically designed around an average starting slag composition and an average desired final slag composition. This inaccuracy is particularly troublesome when attempting to create an optimum foamy slag in the furnace.
In accordance with this invention, there is provided a slag composition comprised of from about 85 to about 99 weight percent of steelmaking slag and from about 1 to about 15 weight percent of a foaming additive. The foaming additive is comprised of from about 20 to about 80 weight percent of a source of elemental carbon, from about 5 to about 80 weight percent of calcium carbide, and from about 25 to about 75 weight percent of an oxide compound. The weight/weight ratio of the source of elemental carbon/calcium carbide is at least 2.
In accordance with another aspect of the present invention, there is provided a method of forming foam in the slag produced during electric arc furnace steelmaking comprising the addition to said slag of an effective amount of a composition comprising from about 20 to about 80 wt % of a carbon source, from about 5 to 80% calcium carbide, the remainder consisting of a mixture selected from the group lime, dolomite, limestone, dolomitic limestone, complex low melting point oxide, byproduct slag, steelmaking slag, and baghouse dust.
In accordance with yet another aspect of the present invention, there is provided a method of creating and maintaining a foamy slag during the earlier stage of the electric arc furnace steelmaking cycle by concurrent addition of a carbon source, an oxygen source, and a complex oxide of melting point less than 2800xc2x0 Fahrenheit (xc2x0 F.).
In accordance with a further aspect of the present invention, there is provided a method of creating and maintaining a foamy slag during the later stage of the electric arc furnace steelmaking cycle by concurrent addition of a carbon source, an oxygen source, and a source of material that reacts endothermicly within the slag and concurrently increases slag basicity.
One aspect of the invention is based on the discovery that the monitoring and control of one or more of several key characteristics related to foamy slag formation leads to more consistent and optimal slag conditions. This discovery avoids problems that arise in conventional processes and results in improved control of chemical and physical properties of the slag that can favorably influence the economics of production and the quality of the final steel product. Hence, an object of the present invention is to provide an optimal method and composition for achieving the desired properties in a steelmaking slag within the electric arc furnace, including the creation of appropriate basicity and chemistry for optimal foaming at each stage of the process. The compositions are designed to combine materials such that the gas formation rate is optimal for sustained efficient foaming at minimal cost. The following classes of materials are used to advantage:
1. Source of oxygen: selected from materials containing iron oxide, or gas containing oxygen, combinations thereof.
2. Source of carbon: selected from carbonaceous materials normally used in metallurgical processes including calcium carbide, carbon, petroleum coke, graphite, coal, lignite, anthracite, coke, metal carbides, and mixtures thereof.
3. Source of slag basicity raiser: selected from calcium carbide, calcium oxide containing materials, calcium carbonate containing materials, magnesium oxide containing materials, magnesium carbonate containing materials, baghouse dust, or mixtures thereof.
4. Source of slag fluidizer selected from silicon oxide containing materials, low melting point complex oxides, waste oxide materials, recycled steelmaking slag, or mixtures thereof.
The present invention is, therefore, intended to make effective use of available real-time and historical information to apply the required reagents in optimum ratios and at optimum addition rates during the melting process for each batch of steel. Depending upon specific conditions at each electric arc steel melting installation, it may be appropriate to manufacture in-situ a slag in more than one step through more than one addition of mixtures of the above materials.
It is a further object of the present invention to optimize the choice of injected agents at each phase of the steel making process to create the desired foamy slag effect reproducibly under all conditions and at least cost. As such, the continuous measurement of slag foaming level is required, as well as a real time knowledge of the stage of the electric arc furnace steel making process. The feed materials to the furnace are also monitored and the melt in slag chemistry is estimated. Thus, the adjustment to this chemistry can be made by correct choice of injected materials in order to start foaming early.
One disadvantage of injected carbon or carbonaceous materials as slag foaming agents is that they are not efficient. Calcium carbide is a more efficient source of carbon for slag foaming, since its reaction rate is slower and it is better wetted by steelmaking slag. On the other hand, calcium carbide is more expensive than injectable carbon. Hence, an optimal blend of calcium carbide and carbon proves will be cost effective while increasing foaming efficiency. The present invention is directed to the optimal composition and a process for achieving and maintaining that composition during the steel making process. Furthermore, calcium carbide is a source of calcium which can reduce the iron oxide content of the slag, thereby improving steel yield. The released calcium can also be effective in sulfur and phosphorus removal. These effects make it particularly attractive to use a higher fraction of calcium carbide near the end of the steel making process just prior to tapping.